The present invention relates to additives for oil-based drilling muds such as are employed for sinking ground bores on- or offshore to drill for gas or oil deposits, and specifically for thickening such mud systems.
Borehole treatment compositions are all types of auxiliary liquids which can be employed for drilling for exploration of crude oil or natural gas reservoirs. In this context, the so-called drilling fluids and drilling slurries or drilling muds built up on these, which can be used both for land-based and for sea-based boreholes, are of particular importance. On the one hand, water-based drilling muds are known with a content of about 1 to 50% of emulsified oily phase—in addition to the other conventional auxiliary substances of such a drilling mud—which are also called O/W emulsion muds. On the other hand, oil-based mud systems in which the oil forms the flowable phase or at least a substantial proportion of the flowable phase as the continuous oily phase, are widely used in practice. The so-called invert drilling slurries which contain, on the basis of W/O emulsions, a disperse aqueous phase in the continuous oily phase are of particular importance here. The content of disperse aqueous phase is conventionally in the range of from at least about 5 to 10 wt. % up to about 50 to 60 wt. %. In addition to these W/O invert drilling slurries, however, the so-called true-oil muds are also known, the liquid phase of which is formed virtually exclusively from a continuous oily phase, and which contain at most small amounts—conventionally not more than about 5 to 10 wt. %—of dispersed aqueous phase.
The non-aqueous phase of such mud systems is formed by the so-called carrier fluid. This was originally diesel oil, which, when treated with certain additives, forms the actual drilling mud. At the end of the 1980s, however, the demand arose for environment-friendly mud systems and therefore carrier fluids. In particular, these had to have a better biodegradability than the diesel oil employed hitherto.
In addition to various liquid esters such as are described e.g. in EP 0 374 672 A1, olefinic hydrocarbons have also been investigated for their usability. Reference may be made here by way of example to EP 0 765 368 A1, which relates to the use of so-called alpha-olefins as the carrier fluid.
In practice, drilling muds for ground exploration are dispersions of a liquid and a solids phase, a considerable solids content being present in the drilling muds. These are liquid mud systems for sinking rock boreholes, with the cuttings detached being brought up. It is therefore necessary for the drilling muds to have a certain density (as a rule greater than 1.2 g/cm3, preferably greater than 1.5 g/cm3), in order to be able to prevent any cutting into the formation. For this purpose, solids are added to the mud for weighting. As a rule, barium sulphate is used. Such liquids as a rule show thixotropic properties under exposure to shearing forces, i.e. the viscosity of these systems, which are also called non-Newtonian fluids, decreases under the influence of increasing shear stress or shear gradient. These properties can lead to problems in practice if the liquids are to be transported or pumped and are thereby exposed to various mechanical stresses. Additives are moreover used as thickeners which increase the viscosity of the mud to facilitate removal of the cuttings.
A large number of additives for increasing the viscosity of oil-based liquids, for example in engine oils or lubricants, are known from the prior art. As a rule, polymer-based thickeners are employed for this. When such additives are employed in the field of drilling muds, however, increased requirements are imposed on these thickeners. When a drilling mud is employed in practice, its rheological properties change continuously, for example due to the nature and amount of the cuttings, or due to cutting in of liquid or gas from the rock formation surrounding the borehole. The additives must moreover be liquid in a wide temperature range, those additives which can also still be used at low temperatures of significantly below 0° C. being sought in particular. The use of these additives of course should not influence the stability of the drilling mud system. The additives should furthermore withstand very high pressures and exposure to high shearing forces and meet the biodegradability requirements increasingly imposed on such systems. In particular, the drilling muds and the additives they contain must not be toxic, this applying in particular for marine life forms, since the muds are preferably used in offshore boreholes.